Severe Hydrothermal Treatment Research Articles

The role of Si coordination structures in the catalytic properties and durability of Cu-SAPO-34 as NH3-SCR catalyst for NOx reduction

Si coordination structures have been proven to greatly influence the ammonia-selective catalytic reduction (NH3-SCR) catalytic properties and the hydrothermal stability of Cu-based silicoaluminophosphate-form catalysts. However, the role of various Si coordination structures in the NH3-SCR reaction over Cu-SAPO-34 catalyst remains unknown. Herein, a batch of Cu-SAPO-34 samples with various Si contents was synthesized via a one-pot method to study the role of Si coordination structures in the NH3-SCR catalytic properties and hydrothermal stability. Cu/34–2 with the highest proportion of Si(xOAl) (x = 1~3) structures exhibits remarkable durability with 90% NO reduction efficiency within 200~450 °C even after a hydrothermal aging treatment at 850 °C. In contrast, Cu/34–1 and Cu/34–4 with the highest proportions of Si(4OAl) and Si(0OAl) structures, respectively, are significantly deactivated by the same hydrothermal treatment. To better understand this phenomenon, the relationship between the Si coordination structures and SCR performance is established using characterization techniques and kinetics measurements. Results reveal that a high content of Si(4OAl) and Si(0OAl) is detrimental to the hydrothermal stability of Cu-SAPO-34 catalyst. However, Si(xOAl) (x = 1~3) structures are conducive to the stabilization of isolated Cu2+, thus enhancing the stability to severe hydrothermal treatment.

A copper-impregnated BEA zeolite for adsorption and oxidation of aromatic species during vehicle cold starts

In this study, we prepared an effective cold-start hydrocarbon (HC) trap by impregnating BEA zeolites with copper. The resulting Cu-impregnated BEA zeolites allowed for effective HC trapping and, more desirably, low-temperature HC oxidation. An optimal Cu content (∼5 wt%) resulted in the highest cold-start test (CST) performance with respect to two representatives HCs, propene and toluene. The preferential adsorption of propene under wet conditions was key to achieving high efficiency (∼79 %). We found that the presence of Cu+ ions allowed the preferential adsorption of HCs, especially propene, and the CuO particles on the exterior surface (∼1−2 nm in size) could oxidize HCs at low temperatures (starting from ∼210 °C). Finally, a severe hydrothermal treatment at 800 °C was used to simulate long-term driving; the CST performance revealed that the adsorption ability for propene was dramatically decreased, whereas those of toluene, along with the oxidation ability, were still preserved.

Selected problems concerning volatile organic compounds emission reduction from thick-veneer pine plywood

Selected problems concerning volatile organic compounds emission reduction from thick-veneer pine plywood. The paper presents results of research conducted on volatile organic compounds emissions, considering two different production parameter sets for thick-veneer pine plywood, manufactured in industrial conditions. Both types of plywood were produced from raw wood material, which was hydrothermally treated under two different variants of parameters (I – 47˚C, 19 h; II – 55˚C, 24 h). Based on the results it was stated that severe hydrothermal treatment of raw wood material (longer soaking time, higher temperature) had impact on reduction of plywood VOC emission rates. Main VOCs emitted from pine plywood were monoterpenes and carbonyl compounds. Of the monoterpenes, α-pinene and 3-carene had the highest emission. Of the carbonyl group of compounds, the highest emission had hexanal and caproic acid.

The superior hydrothermal stability of Pd/SSZ-39 in low temperature passive NOx adsorption (PNA) and methane combustion

Uniform SSZ-39 (Si/Al ratio ∼12) crystals with an average size of about one micron were synthesized and used to support Pd (0.7–3 wt%) for PNA. The as-synthesized materials were characterized by FTIR, XRD, Helium Ion Microscopy, HAADF-STEM imaging, 27Al, 29Si, and 1H-29Si CP solid state NMR spectroscopic techniques. FTIR studies with CO and NO probe molecules reveal that the majority of Pd is dispersed as isolated Pd(II) and Pd(II)−OH centers and as such is suitable as a low-temperature passive NOx adsorber. Pd(II)-NO, Pd(II)(OH)(NO), and Pd(II)(CO)(NO) complexes form in this material during PNA. Comparison to Pd/SSZ-13 (Si/Al ∼12) shows the superior hydrothermal stability of this new material, surviving hydrothermal aging up to 815 °C in 10 % H2O/Air for 16 h without a significant loss of activity. The SSZ-39 crystal structure remains intact during hydrothermal aging up to 1000 °C confirmed by XRD and HAADF-STEM imaging/EDS mapping. However, changes to the framework, as evidenced by high-field 27Al NMR, during such severe hydrothermal treatment significantly alter the NOx release profiles. Besides PNA, this hydrothermally stable material (3 wt% Pd on SSZ-39; Si/Al ∼12) can be used as a robust methane combustion catalyst under practically relevant conditions (i.e., GHSV∼300 L/g*hr). This catalyst shows minimal deactivation after both hydrothermal aging at 750 and 800 °C and prolonged time on stream (105 h) at 425°C. In contrast, both 3 wt% Pd/alumina and 3 wt% Pd/SSZ-13 lose a significant portion of their activity under such conditions, marking an improvement over current technology.

N-doped titanate nanostructures on gradient surface layer of one dimensional nanofibers synthesized on Ti electrode by chemical and thermal treatments

TiO2 nanostructures have attracted much attention due to their semi-conductivity and photocatalytic properties, but are rarely used as functional electrodes for water electrolysis that requires high electrical conductivity. Compared to TiO2, a high doping level of N into titanates is expected due to their layered structure, which was attempted to improve the conductivity. Here, a surface layer of one dimensional hydrogen titanate (H-titanate) nanofibers with high surface area and poor crystallinity was synthesized on the surface of Ti metal by mild NaOH and HCl treatments, as compared to the commonly used severe hydrothermal treatments. The poorly crystallized structure of the layered H-titanate allowed the incorporation of a large amount of N into its surface layer by thermal treatments at 700 °C or more, thereby transforming it into Ti-oxynitrides with high electrical conductivity while retaining the high surface area of the treated layer. The depth profile analyses of the chemically and thermally treated Ti metal revealed that the concentration of O and H was the highest at the top surface of the surface layer. With increasing depth, the concentration of O and H decreased gradually whereas the concentration of N increased gradually, indicating gradients in the structures of the treated nanofibrous surface layer. The dense region below the nanofibrous surface layer was composed of Ti2N, whose amount increased with increasing temperature. This kind of gradient surface layer of N-doped titanate nanofibers without well-defined interfaces was synthesized on the surface of the Ti electrode by our unique chemical and thermal treatments and was modeled with varying depths. The specific objective of the present work is to characterize and study the mechanism of formation of the N-doped structures in a one dimensional gradient surface layer of H-titanate nanofibers synthesized on the treated Ti electrode. The treated one dimensional nanofibrous surface layer on a Ti electrode can be potentially used for the electro-splitting of water after fixation of molecular catalysts on its surface.

Effect of hydrothermal treatment varying in time and pressure on the properties of parboiled rices with different amylose content

Effects of mild, moderate and severe hydrothermal treatments at open and under steam pressure on four rice varieties differing in amylose content were studied. Degree of gelatinization was markedly high in the severely pressure parboiled samples. Pressure parboiled waxy samples showed extensive fall of gelatinization temperature and increased hydration at lower temperatures. Formation of short amylopectin fine structures in these samples was indicated by sediment volume and viscosity patterns. The pattern of change in the infrared spectroscopic bands may be due to shift in stretching mode. Wide angle X-ray diffractographs of raw samples showed peaks at 2 θ values near 20 which indicated ‘in situ’ points for amorphous amylose complex formation. A, B and V-type polymorphs were seen in pressure parboiled samples and only A and V-type polymorphs were observed in open steamed samples. Loss in crystallinity with simultaneous increase in water uptake can be attributed to the amorphous fractions in parboiled rice.

Hydrothermal study of Pt–Sn-based SAPO-34 supported novel catalyst used for selective propane dehydrogenation to propylene

Catalyst's regeneration is unavoidable part during dehydrogenation. The hydrothermal treatment influence on the performance of Pt–Sn-based SAPO-34 supported novel catalyst, used for propane dehydrogenation to propylene is investigated in this study. The catalyst shows excellent stability for mild steaming (nitrogen mixed steam), during four consecutive runs (reaction–regeneration mode). On the other hand, Pt–Sn/ZSM-5 was largely affected on mild steaming due to severe dealumination. In order to get into mechanistic understanding, severe hydrothermal treatment was carried our using pure steam. The substation loss in catalyst activity was noted. Both fresh and severe hydrotreated catalysts were characterized by XRD, XRF, O 2-pulse analysis of coke, NH 3-TPD, IR spectrum of adsorbed ammonia, H 2-TPR, HR-TEM and XPS, to explore reasons for change in catalytic properties. The texture/topology is found stable. Changes in surface ensembles occur due to the loss of Sn, Al, formation of SnO x species and in particular Pt sintering. This leads Pt active sites (zeolite–SnO–Pt) to inactive sites (zeolite–Pt, zeolite–PtO–Sn, Pt–Sn alloy, etc.) formation and reduced catalyst activity. TEM micrographs and H 2-chemisorption analysis confirms Pt particles agglomeration and/or sintering. About 98% catalyst activity is recovered by re-dispersed Pt using chlorination technique and credit goes to hydrothermally stable support (SAPO-34).

Effect of Hydrothermal Treatment on the Acidity and Catalytic Performance of Nanosized HZSM-5 Zeolites for the Conversion of Methanol to Propene

A series of modified nanosized HZSM-5 zeolites were prepared by hydrothermal treatment at different temperatures and characterized by the adsorption and desorption of N2,X-ray diffraction (XRD),Al solid state magic angle spinning nuclear magnetic resonance (27Al MAS NMR),temperature-programmed desorption of ammonia (NH3-TPD),and Fourier transform infrared (FT-IR) spectroscopy of adsorbed pyridine techniques. The conversion of methanol to propene was tested using a continuous flow fixed-bed microreactor at atmospheric pressure,500 ℃,and a methanol weight hourly space velocity (WHSV) of 1.0 h-1. Results showed that after the appropriate hydrothermal treatment,some of Al was removed from the framework of zeolite and extracted out of the zeolite channels after treatment with an aqueous solution of citric acid. This led to a decrease in the strength and amount of acidic sites and an increase in the volume and diameter of the pores. As a result,propene selectivity and the reaction time continued to maintain the total conversion of methanol (i.e.,life time of catalyst) increased significantly to 38.9% and 160 h from 30.1% and 75 h over the parent nanosized HZSM-5 zeolite. However,severe hydrothermal treatment decreased the acidity remarkably and the strongly acidic sites were almost totally destroyed,resulting in a remarkable decrease both in propene selectivityand catalyst stability.

Impact of parboiling conditions on Maillard precursors and indicators in long-grain rice cultivars

The effect of steaming conditions (mild, intermediate and severe) during parboiling of five different long-grain rice cultivars (brown rice cultivars Puntal, Cocodrie, XL8 and Jacinto, and a red rice) on rice colour, and Maillard precursors and indicators was investigated. Rice colour increased with severity of parboiling conditions. Redness increased more than yellowness when parboiling brown rice. Parboiling turned red rice black. It changed the levels of glucose, fructose, sucrose, and maltose. Losses of the non-reducing sugar, sucrose were caused by both leaching into the soaking water and enzymic conversion, rather than by thermal degradation during steaming. Concentrations of the reducing sugars, glucose and fructose, in intermediately parboiled rice were higher than those of mildly parboiled rice. After severe parboiling, glucose levels were lower than those of intermediately parboiled rice, while fructose levels were higher. These changes were ascribed to the sum of losses in the Maillard reaction (MR), formations as a result of starch degradation and isomerisation of glucose into fructose. It was clear that the ε-amino group of protein-bound lysine was more affected by parboiling conditions and loss in MRs, than that of free lysine. Low values of the MR indicators furosine and free 5-hydroxymethyl-2-furaldehyde (HMF) in processed brown and red rices were related to mild parboiling, whereas high furosine and low free HMF levels were indicative of rices being subjected to intermediate processing conditions. High furosine and high free HMF contents corresponded to severe hydrothermal treatments. The strong correlation (r=0.89) between the free HMF levels and the increase in redness of parboiled brown rices suggested that Maillard browning was reflected more in the red than in the yellow colour.

Hydrothermal stabilization of ZSM-5 catalytic-cracking additives by phosphorus addition

ZSM-5 zeolites with different Si/Al ratios (15, 25, 40) were impregnated with phosphorus (0.5–3 wt%) in form of H 3PO 4 or NH 4H 2PO 4. The samples were characterized before and after severe hydrothermal treatment by XRD, IR, 27Al, 29Si, 31P MAS NMR, and their activity for the cracking of n-decane was measured. It was found that phosphorus impregnation increases the hydrothermal stability of framework aluminum whatever the source of phosphorus used. The acidity and cracking activity of steamed samples reached an optimum for a P/Al molar ratio of ca. 0.5–0.7. A chemical model for the phosphorus–zeolite interaction is proposed in which the framework aluminum pairs are stabilized by extra-framework cationic species formed by protonation of orthophosphoric acid. The influence of isolated versus pairs of aluminum on activity and selectivity after steaming is discussed. When P-impregnated ZSM-5 sample is used as an additive for cracking industrial feeds, selectivity to propylene and butenes increases.

Performance of New Pt and Pt–Cu on Hydrotalcite-Derived Materials for NOxStorage/Reduction

A series of Pt and Pt,Cu supported catalysts was prepared by impregnation of Mg–Al supports obtained from hydrotalcite-type (HT) precursor compounds calcined at different temperatures. These NOx storage-reduction (NOxSR) catalysts show improved performances in NOx storage respect to Pt–Ba/Al2O3 NOxSR catalysts at reaction temperatures lower than 473 K. These catalysts also show improved resistance to deactivation by SO2. The co-presence of Pt and Cu (at different ratio Pt/Cu and using different impregnation methods) slightly worsens low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in presence of SO2. The overall Pt–Cu/HT performances are thus higher than those of the Pt–Ba/Al2O3 Toyota-type NOxSR catalysts.

Argon and nitrogen adsorption studies of changes in connectivity of ordered cage-like large mesopores during the hydrothermal treatment

FDU-1 silicas with large cage-like mesopores were synthesized, using tetraethyl orthosilicate as a silica source and a poly(ethylene oxide)–poly(butylene oxide)–poly(ethylene oxide) triblock copolymer (EO 39BO 47EO 39) as a structure-directing agent. The evolution of the FDU-1 pore structure during a hydrothermal treatment at 373, 393, and 413 K was studied, using nitrogen and argon adsorption at 77 K. As the time of the hydrothermal treatment was extended, a moderate increase in the pore cage diameter and a significant enlargement of the pore entrances took place, as inferred from the analysis of adsorption and desorption branches of isotherms, respectively. The materials retain a monodisperse distribution of pore cage diameters. On the other hand, argon and nitrogen desorption data provided evidences that there are two major populations of pore entrances according to their diameter and that they both undergo a concomitant enlargement as the hydrothermal treatment is prolonged. Argon desorption was superior in providing information about pore connectivity in FDU-1 samples, except for those obtained after particularly severe hydrothermal treatments (long time or high temperature). The latter materials had a fraction of pores, which were so large that the capillary condensation of argon at 77 K in them was not observed, and consequently, the information regarding the connectivity and volume of this fraction of pores cannot be extracted from the argon data. In the case of relatively large mesopores (above ∼15–20 nm), nitrogen that exhibits capillary condensation at 77 K in much wider range of pore sizes (up to 100–200 nm) is better as far as the pore connectivity assessment from desorption data is concerned.

Microporosity and connections between pores in SBA-15 mesostructured silicas as a function of the temperature of synthesis

Imaging of the platinum replica of the porous structure and low-pressure argon adsorption allowed us to elucidate the complicated porous structure of SBA-15. These techniques enabled us to draw a coherent picture of the evolution of the SBA-15 precursor mesophase as a function of the synthesis temperature. TEM of the platinum replicas has been unable to show bridges between the structural mesopores of SBA-15 synthesized at low temperature, whereas mesoporous bridges are clearly observed for samples formed at higher temperature. Argon adsorption has evidenced the ultramicroporosity of the materials formed at low temperature, as well as its evolution to secondary porosity with diameters greater than 1.5 nm under more severe hydrothermal treatment.

A New Templated Ordered Structure with Combined Micro- and Mesopores and Internal Silica Nanocapsules

The increasing awareness of the need to create green and sustainable production processes in all fields of chemistry has stimulated materials scientists to search for innovative catalysts supports. These new catalytic supports should allow the heterogenization of most catalytic processes, increasing the efficiency and selectivity of the synthesis and reducing waste and byproducts. Following the development of several micellar templated structures, such as M41S1, FSM-16,2,3 HMS,4,5 MSU-x,6 and SBA-x,7,8 it is a crucial next step to create support materials, consisting of a composite matrix with combined micro- and mesoporosities and a sufficient stability to withstand most industrial treatments. We describe in this paper the very first development of a hexagonal material with large pore diameters and thick walls (4 nm), containing internal microporous silica nanocapsules. These plugged hexagonal templated silicas (PHTS) have two types of micropores (originating from the walls and the nanocapsules respectively) and a tunable amount of both open and encapsulated mesopores. The micropore volumes have a high value (up to 0.3 cm3/g) and the total pore volume exceeds 1 cm3/g. The obtained materials are much more stable than the conventional micellar templated structures known so far, and can easily withstand severe hydrothermal treatments and mechanical pressures.

NO x storage-reduction catalysts based on hydrotalcite : Effect of Cu in promoting resistance to deactivation

Novel NO x storage-reduction (NO x SR) catalysts prepared by Pt and/or Cu impregnation of Mg–Al (60:40) hydrotalcite (HT)-type compounds show better performances in NO x storage than Pt–Ba/Al 2O 3 Toyota-type NO x SR catalysts at reaction temperatures lower than 250 °C. The presence of Pt or Cu considerably enhances the activity, with the former more active. The nature of the HT source, however, also influences performance. The co-presence of Pt and Cu slightly worsens the low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in the presence of SO 2. This effect is attributed to both the possibility of formation of a Pt–Cu alloy after reduction, and the modification of the HT induced during the deposition of Cu. The overall Pt–Cu/HT performances are thus superior to those of the Pt–Ba/Al 2O 3 Toyota-type NO x SR catalysts.

Surface acidic properties of alumina-supported niobia prepared by chemical vapour deposition and hydrolysis of niobium pentachloride

Niobia–aluminas were prepared by chemical vapour deposition at 150°C of niobium pentachloride on the surface of γ-aluminas calcined at different temperatures and with controlled degrees of hydration, followed by hydrolysis with water vapour at 150°C and a thermal treatment with steam at 440°C aimed at removing surface chloride contamination. The samples were characterised with respect to chemical composition, surface area, acidity by temperature-programmed desorption of ammonia, nature of acid sites by infrared spectroscopy of adsorbed pyridine and catalytic activity at 370°C in the dealkylation of cumene. The results showed that, for each alumina calcination temperature, the catalysts with the lowest niobium content have a higher density of acid sites than the alumina support, but the acidity decreased, within each series with an increase in the niobium content. Comparatively to the TPD results, catalytic activity in cumene dealkylation was much more sensitive to the history and composition of the samples. The niobia-alumina samples were much less active than the alumina support, but this was most likely due to the severe hydrothermal treatment for chlorine removal, since their activity was close to that of an alumina submitted to the same treatment. A strong decrease in the acidic activity was observed with increase in the niobium content. A sample of pure niobium oxide had a much higher activity than the niobia-alumina samples. Brønsted acidic sites could only be observed by the IR spectra of adsorbed pyridine on the surface of the pure niobium oxide sample.

Catalytic cracking studies and characterization of steamed Y and LZ-210 zeolites

Catalytic cracking catalysts prepared using Y zeolite crystals which have been silicon enriched by treatment with ammonium fluorosilicate solution show substantially superior crystal retention upon exposure to severe hydrothermal treatments, relative to similarly treated Y-zeolite-based catalysts. The silicon-enriched Y-based catalysts, following hydrothermal pretreatment, also show superior activity retention and higher gasoline selectivity relative to Y-zeolite-based catalysts. The study of steamed Y and the silicon-enriched Y product by solid-state MAS NMR shows that upon steaming the silicon-enriched zeolite displays superior retention of the framework structure relative to Y, and that the Y product shows not only aluminum loss but also significant silicon loss from the crystal framework, contributing to deterioration of the crystal. Thermometric titrations of Y zeolites, following exposure to steam treatments of increasing severity, indicates that upon severe steam treatment the strong acid character typical of stabilized Y is greatly reduced while a substantial concentration of milder acid sites is formed, similar in acid strength to amorphous silica-alumina gel. These data are interpreted on the basis that upon steaming amorphous alumina and silica-alumina phases are formed, occluded in the zeolite crystal, and that the occluded amorphous, acidic debris adversely affects cracking selectivity to produce gasoline.